The present invention relates to a method and an apparatus for testing through known penetration methods the hardness of a workpiece having a continuously curved outer surface, such as pipes or rods.
Depending on the use, a workpiece made of for example steel, and manufactured by various methods, must in many cases meet minimum toughness standards. Compliance with these standards can be ascertained by chemical analysis, observation of the nature and amount of deformation, with subsequent optional heat treatment, or a combination thereof. In this regard, the heat treatment may be an integral part of the method of manufacture or constitute a separate step within a sequence of different operating steps. In order to be able to test for compliance with the required minimum hardness not only during the course of the final acceptance testing, but during other relevant phases, the result of the heat treatment should be checked in accordance with the principles of controlled manufacture by a hardness test which is carried out on each piece or in accordance with a random sampling plan.
By means of known reference tables, the hardness value found can then be associated with a corresponding hardness or tensile strength value. Since hardness measurements should be taken, when possible, during the course of manufacture, a test station having a hardness testing instrument is arranged behind the heat-treatment unit. Depending on the cooling conditions, the test station can be located either immediately behind the heat-treatment unit or somewhat further removed therefrom. The test station consists essentially of a holding device which fixes the workpiece in position, a device which bears the working tool, and a hardens testing instrument. The Brinell method or the Vickers method is used, with perference being given in most cases to the Brinell method. The test method is carried out in such a manner that, after the workpiece has been fixed in position on the roller table, a point on the surface of the workpiece which lies within the test region is machined in order to produce a flat and smooth test surface. The machining of the test surface is done by milling or grinding with a predetermined depth. The pressing of the penetration body into the test region, through the use of a ball or a pyramid, is done with a defined force, and in the ideal case, precisely within the central region of the machined surface. The imprint of the resultant impression area is transferred in a known manner by means of a swingable optical system onto a screen. The examining person can then determine the size of the penetration surface with the aid of known means by measurement of its diameter or diagonal. This area then constitutes, with respect to comparative values of reference pieces, a measure of the hardness of the workpiece tested. In order to carry out this known method substantially automatically, it has already been proposed that a test laboratory, by a contact-free manner, scan the surface impression produced by the ball on the surface of the workpiece by means of a camera (see in this connection the prospectus of Foundrax Engineering Products Limited). The camera is connected to an evaluation unit which, by means of known mathematical algorithms, converts the measured value into the corresponding hardness value.
From Federal Republic of Germany 39 01 432 it is furthermore known that a determination of the ball indentation diameter can be made with sufficient accuracy on a Brinell hardness testing machine, by means of a video camera, a digitizer and a computer, as well as suitable software.
One testing device of this type which is suitable for the testing of the hardness of pipes or rods is known from U.S. Pat. No. 4,635,471. A vertically moveable mount having a V-block for the workpiece as well as a horizontally and vertically displaceable device, on which a machining tool and a hardness testing instrument are fastened, are arranged on a rack. The workpiece is fixed in position on the V-block by clamping holders which are displaceable in guide rails and the displaceable arrangement is then positioned on the axis of the pipe. This testing device has the disadvantage that a lateral and/or vertical offset of the testing place with respect to the axis of the V-surface resulting from a curved and/or oval pipe cannot be automatically compensated for and machining, for instance milling, may in the extreme case be effected too deep or not at all and, therefore, the predictive power of the values obtained at different depths of material is limited.
JP 70-143739 A discloses an automatically operating hardness testing instrument in which the picture contour is recognized by a sensor which scans the test piece and a test locus, the coordinates of which have been previously established, is automatically found by comparison with a desired picture which is stored in a computer. For this purpose, the resting surface for the test piece is displaceable in x and y directions. By this method it is intended to provide assurance that the same point is always tested for a number of identical test pieces. Furthermore, with this method the hardness penetration can be placed in a region which does not limit the use of the workpiece.